A Study of Novel Agents or Combinations as Perioperative Treatment in Participants With Locally Advanced Resectable Gastroesophageal Adenocarcinoma

GEMINI-PeriOp GC study will assess the safety, tolerability, pharmacokinetics (PK) and preliminary anti-tumor activity of novel agents or novel combinations as perioperative treatment in participants with locally advanced resectable gastric, gastroesophageal junction (GEJ), or esophageal adenocarcinoma who have not received previous treatment for the disease.

Inclusion Criteria: * Histologically documented gastric, GEJ, or esophageal adenocarcinoma with resectable disease * Participants who are CLDN18.2-positive in Sub-study 1 or HER2-positive in Sub-study 2; no specific requirements for Sub-study 3 * Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 * Adequate organ and bone marrow function * Body weight \> 35 kg Exclusion Criteria: * Participants had any prior anti-cancer treatment or surgery for the current gastric, GEJ, or esophageal cancer. * Active or prior documented autoimmune or inflammatory disorders requiring systemic treatment with steroids or other immunosuppressive treatment * Central nervous system (CNS) pathology * Uncontrolled infections * Participants with history of (non-infectious) interstitial lung disease (ILD)/pneumonitis, current ILD/pneumonitis, or suspected ILD/pneumonitis * History of another primary malignancy * Participants with any known or suspicious distant metastasis * Uncontrolled hepatitis B and/or chronic or active hepatitis B * Current or prior use of immunosuppressive medication within 14 days before the first dose of study intervention